This invention relates to time division multiplexers and more particularly to methods for synchronizing such multiplexers.
Multiplexers are devices which permit information to be transmitted from a plurality of information sending devices to a plurality of information receiving devices over a single aggregate line. Communicating multiplexers typically transmit frames of data and control information from a plurality of channels as well as intermultiplexer overhead information according to a frame format. A frame is a fixed number of sequentially transmitted bits. A frame format is the order in which the bits (which are typically received from a plurality of sources) are arranged for transmission. The meaning or significance of each bit may therefore be determined from its position within the frame format. Thus, the transmitting multiplexer places the data and control information from each channel into the allocated bit positions according to the frame format. Intermuliplexer overhead information is likewise added to the frame and the frame is serially transmitted to the receiving multiplexer. Because the information is transmitted in a particular order according to the frame format, the receiving multiplexer can determine whether the received information is to be sent to a particular channel or whether it is to be used internally. The information to be used internally is the intermultiplexer overhead information and includes intermultiplexer communications information and intermultiplexer synchronization information. The intermultiplexer synchronization information permits the multiplexers to work in synchronization such that the data, control, and other information which was combined into a single bit stream according to the frame format of the sending multiplexer, is properly decoded by the receiving multiplexer so that it is properly sent to its destination.
A conventional technique for achieving synchronization between multiplexers is to transmit a synchronization word at the start of each frame. A synchronization word consists of a fixed number of bits that have been set to known values. The synchronization bits are sent by a first multiplexer at the start of the frame, and at a second or receiving multiplexer (demultiplexer), the incoming bits are scanned until the synchronization word is found. Since it is possible for data bits to assume the same values as the bits in the synchronization word, the corresponding bits in several successive frames must hold a valid synchronization word before the receiving multiplexer can safely assume that frame synchronization has been achieved.
While the conventional synchronization technique typically performs its function properly, in certain circumstances, such as where a channel is continuously transmitting tones at particular amplitudes and frequencies, the synchronization word can be imitated by data bits of several frames. Thus, the receiving multiplexer could find synchronization at the wrong point. Another problem with the conventional technique is that the synchronization bits must be included in the frame because they are required to first establish synchronization. As a result, the synchronization bits cannot be used to convey any information other than the frame format position. However, once synchronization has been achieved, the synchronization bits are almost always redundant. This redundancy thereby decreases the possible data throughput of the system.